JPS63210063A - Zirconia oxygen sensor element - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to an oxygen sensor element used in an environment with severe temperature changes, and particularly provides a zirconia oxygen sensor element with improved thermal shock resistance. It is.

[Conventional technology]

Zirconia ceramics are preferably used as oxygen sensor elements in automobile exhaust gas and steel melting furnaces because of their high oxygen ion conductivity and large mechanical strength. The usage conditions for such oxygen sensor elements are often accompanied by rapid thermal changes1.For example, oxygen sensors for automobile exhaust gas and gas require harsh thermal environments ranging from approximately 900°C to -20°C (in cold regions). It is placed below. Therefore, this element has
In addition to the above two properties, high thermal shock resistance is required.

As is well known, zirconia (ZrOz) changes into monoclinic (~1100℃) and tetragonal (1100℃) depending on the temperature.
00~2300℃), cubic crystal (2300~2700℃)
There is a crystal phase of about 9% during the monoclinic/tetragonal transformation. Because of this large volume change, self-destruction occurs during the cooling stage during sintering, resulting in pure (monoclinic) crystal phase. It is difficult to obtain a zirconia sintered body. Therefore, calcia (Cab), magnesia (MgO), ittria (Y
Usually, a predetermined amount of zOa) or the like is added to make the cubic crystal, which is originally a high-temperature phase, stably exist at room temperature. This cubic zirconia has oxygen ion conductivity, and because of this property, it is used as an oxygen sensor element. However, cubic zirconia has a thermal expansion coefficient of 11~12XIO-'℃
-'' (20-1500°C), which is relatively large among inorganic materials, and under conditions of rapid heating and cooling, it cannot withstand the thermal stress generated at that time, and cracks easily propagate, causing cracks in the element.

Therefore, in order to compensate for this drawback, Japanese Patent Application Laid-Open No. 59-4195
As described in Japanese Patent No. 2, an appropriate amount of monoclinic crystal is mixed with cubic crystal to lower the thermal expansion coefficient of the element itself, thereby improving thermal shock resistance. Furthermore, the zirconia crystal grains in the sintered body are refined as described in JP-A-56-111456.

Techniques for improving element strength and thermal shock resistance have also been published.

(Problems to be Solved by the Invention) Among the above-mentioned conventional techniques, in the method of dispersing monoclinic zirconia particles in a sintered body of neutral cubic zirconia, a considerable amount of monoclinic zirconia, which originally has poor sinterability, is removed. As a result, the sintering temperature increases, the oxygen ion conductivity decreases, and there is a risk of element destruction during firing and cooling.Also, the volume change due to the monoclinic-tetragonal transformation When used in a plug body like an oxygen sensor element, the joints may become loose, which is undesirable.On the other hand, with the technology of making cubic zirconia particles finer, the strength is slightly higher. However, there is no essential change in the coefficient of thermal expansion, and therefore no effect of improving thermal shock resistance can be expected.

In other words, with regard to the thermal shock resistance of zirconia as an oxygen sensor element, the conventional technology, for example, causes a large change in the characteristics of the zirconia sintered body, and although it is effective in increasing the strength, it does not take thermal shock resistance into consideration. The problem was that it had not been done.

[Means for solving problems]

The present inventors conducted various studies in order to obtain a zirconia oxygen sensor element with excellent thermal shock resistance, and found the following fact. That is, the tetragonal zirconia is transformed into monoclinic zirconia by applying an appropriate heat treatment, and the depth of this transformation is approximately 50 μm from the surface of the sintered body. When we investigated the characteristics of a zirconia sintered body whose surface had changed to monoclinic zirconia, we found that it had increased thermal shock resistance compared to a zirconia sintered body whose surface had not changed in any way.

Therefore, by applying a monoclinic zirconia layer to the surface of a zirconia oxygen sensor element using one of various methods, it was possible to provide a higher thermal shock resistance than the conventional zirconia oxygen sensor element. That is, in the present invention, any crystal phase of zirconia used in the zirconia oxygen sensor element may be used.
For example, even with stabilized zirconia consisting only of cubic crystals,
In addition to cubic crystal, it may contain crystal phases such as tetragonal, monoclinic, and rhombohedral, as long as the surface contains 10% or more more monoclinic than the inside. .

In order to obtain the zirconia oxygen sensor element of the present invention, the following method was used. Zirconia raw material powder in which 2 to 6 mo 0% of a stabilizer such as Y z Oa is dissolved is granulated by a known method such as spray drying.

The sensor element is formed into a bag tube shape using hydrostatic pressure, etc. After sintering this molded body at 1400 to 1550°C, platinum electrodes were formed on the inner and outer surfaces of the element by plating.

Aging is performed in the atmosphere at 100 to 350°C for 10 hours or more.

This aging transforms the tetragonal zirconia on the element surface into monoclinic zirconia. Alternatively, zirconia green sheets are laminated as appropriate to make a flat oxygen sensor element. On top of this, monoclinic zirconia green sheets were laminated from above and below by thermocompression, and then heated to 1400 to 1550°C.
It can be obtained by sintering.

The zirconia raw material powder used to obtain such oxygen sensor elements contains a small amount of hafnia (HfOz) and titania (Ti).
Therefore, the inclusion of these impurity components in the zirconia oxygen sensor element of the present invention is allowed.

Furthermore, alumina (A Q zoa
) and silica (Si, Oz), etc., but these components do not affect the effects of the present invention at all.

[Function] The reason why the thermal shock resistance of the sensor element is improved by depositing a monoclinic zirconia layer on the surface is considered to be as follows. As mentioned above, zirconia undergoes a volume change of about 9% when it transforms from monoclinic to tetragonal, but this is because monoclinic zirconia particles are larger than tetragonal zirconia particles. . That is, precipitating monoclinic crystals on the surface of the element produces a compressive stress field on one surface. For example, when a device is subjected to a thermal shock such as quenching in water, a large tensile stress is generated on the surface of the device, which causes cracks, which propagate and eventually lead to destruction of the device. However, when compressive stress is applied to the surface as in the present invention, the tensile stress caused by this thermal shock is alleviated. Therefore, it is difficult for cracks to form and the thermal shock resistance is improved.The thickness of the monoclinic zirconia layer is preferably 2cm or less;
The layer is too thick and self-destructs as the volume changes, making it impossible to form one surface layer.Also, the monoclinic content of the first surface layer was set to be 10% or more than the interior because if it was less than this, there would be sufficient compressive stress. This is because the effect of thermal shock resistance cannot be expected.

Regarding the manufacturing method, when an element containing tetragonal zirconia is transformed into monoclinic zirconia by aging, the temperature range is limited to 350°C even if it is below 100°C.
Even with the above, the tetragonal crystals in the surface portion remain stable, and the effect of the present invention cannot be obtained.

〔Example〕

<Example 1> (1) Three types of zirconia powders (manufactured by Toyo Soda Kogyo Co., Ltd., purity 99.9%, average particle size 0.03 μm) with different degrees of stabilization of Y2O3 synthesized by the neutralization coprecipitation method (YzO
A (stabilization degree of 3%, 5%) was prepared, formed into a bag tube shape using a rubber press (pressure 68.6 MPa) and ground, and then sintered at 1500° C. for 1 hour in a tunnel furnace. Platinum electrodes were formed on the inner and outer surfaces of this bag tubular element by electroless plating. The element obtained in this way is placed in an electric furnace.

It was incubated at 250°C for 100 hours.

The following thermal shock test was conducted on these samples.

Test 1: A bag tubular sample was placed in a burner flame at 850° C. for 30 seconds, and then rapidly cooled while blowing cold air.

Afterwards, we sprayed dyed penetration wounds for nuclear equipment made by Eishin to investigate the occurrence of cracks.

Test 2: A sufficient amount of the bag tubular sample was kept in an electric furnace at 500°C and then dropped into water. Thereafter, the presence or absence of cracks was examined using the same method as Test 1.

Test 3: The bag tubular sample was heated in an electric furnace from room temperature to 1000°C in 2 hours, held for 30 minutes, then the heater was turned off.
Cool in the oven to 350°C in 5 hours. The temperature is then raised to 1,000 degrees Celsius again. Repeat this cycle five times, then test 1
The presence or absence of cracks was examined using the same method.

The results of the sample obtained in Example 1 and Tests 1 to 3 are summarized. The crystal phase was identified by X-ray diffraction, and the thickness of the surface layer was determined from X-ray diffraction and a SEM photograph.

From Table 1, the thermal shock resistance of the sample subjected to aging treatment and with a large amount of monoclinic zirconia precipitated on the surface layer is as follows.

All samples have improved. Note that it is the tetragonal particles on the surface that are transformed into monoclinic crystals, and this causes a compressive stress field to occur on the element surface.

Figure 1 shows a simulated cross-section of the element.Also, in order to generate a compressive stress field on the surface, a technique for bonding materials with a coefficient of thermal expansion smaller than that of zirconia can be considered, but in such a method, for example, When heat cycles such as those in Test 3 are applied, there is a high possibility that peeling or the like will occur. However, the present invention is characterized in that it utilizes the volume change of similar substances, and no peeling or the like was observed in the sample after Test 3.

<Example 2> A green sheet was created using zirconia powder containing 3 mo 0% of Y2O3. After printing platinum electrodes on this green sheet, it was cut into strips as shown in Fig. 2, laminated by thermocompression, and 1
Sintering was carried out at 500°C for one hour. After that, 250℃
The element was aged at 0. The element was then kept in an electric furnace set at a predetermined temperature for 20 minutes, and then dropped into water to measure the four-point bending strength of the element. Thermal shock resistance was then evaluated at a temperature at which the bending strength was drastically reduced compared to before being dropped into water. The results are shown in FIG. 2 and Table 2. In FIG. 1, the horizontal axis represents the predetermined temperature, and the vertical axis represents the room temperature bending strength of the element after being dropped into water from the temperature indicated on the horizontal axis.

The sample numbers indicate the difference in aging time between those aged at 250°C and those aged at 250°C, and are shown in Table 2.

Table 2 From Figure 2 and Table 2, we can see that there is a sample with no aging process (&1) and a sample with a strong aging time of 5 hours (&1).
In case 2), when dropped into water from 200°C, the initial strength was not maintained at all, and the strength was drastically reduced by only a few MPa. On the other hand, the samples (&3 to 5) that were subjected to aging for more than 10 hours maintained the strength at the beginning of C up to around 300°C, indicating that the thermal shock resistance was improved. FIG. 3 shows the relationship between aging time and monoclinic content on the surface. If the number of monoclinic crystals on the surface of this figure increases by 10% or more compared to the inside, it can be said that the thermal shock resistance of the element improves.

<Example 3> A green sheet was prepared from zirconia powder containing 6 mol % of Y2O3, and the strip-shaped sensor element shown in FIG. 3 was made. Furthermore, Green Sea 1- was made from monoclinic zirconia powder, and a predetermined number of sheets were laminated from above and below this element. The unfired oxygen sensor element thus produced was sintered at 1000°C for one hour to obtain an oxygen sensor element with a monoclinic layer formed on the surface.Table 3 shows the thickness of the monoclinic layer after sintering. It shows the presence or absence of cracks in the monoclinic layer.

Table 3 As seen from Table 3, the thickness of the monoclinic layer is 1 m or less (N
Although no cracks occur in αl-4).

If this value is exceeded (Nα5), a large rack will occur on the 1 surface portion, and a healthy oxygen sensor element will not be obtained.

It is thought that when the monoclinic layer becomes thicker, the number of zirconia particles increases accordingly, and the effect of volume change during sintering and cooling becomes too large, resulting in self-destruction. In addition, the thermal shock resistance of the samples &1 to Na4 in which the monoclinic layer was formed was all good when the thermal shock test of Example 2 was conducted.

<Example 4> Oxygen sensor element (Y
zOa5mo Q% containing zirconia) was aged for 50 hours at various temperatures. Then, the monoclinic crystal on the surface of the 1M element is
It was investigated by line diagram folding. Thereafter, a thermal shock test was conducted using the method of Test 2, and the results are shown in Table 4.

As is clear from Table 4, the aging temperature was less than 100°C (Sample Na1) and above 350°C (Sample Na7).
), it can be seen that the amount of precipitated monoclinic crystals is small and the thermal shock resistance of the device is not improved. From this result, it can be said that the optimum aging temperature is 100 to 350°C.

〔Effect of the invention〕

According to the present invention, an oxygen sensor with high thermal shock resistance can be obtained, so that it can cope with harsher measurement atmospheres.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view near the surface of the zirconia oxygen sensor element prepared in Example 1, and FIG. Figure 3 shows the results of the thermal shock resistance test conducted in Example 2. Figure 3 is an assembly diagram of the strip-shaped oxygen sensor element prepared in Example 2.3. Figure 4 shows the aging time conducted in Example 2. FIG. 3 is a diagram showing the relationship between the monoclinic crystal content of the surface portion and the monoclinic crystal content of the surface portion. 1... Cubic zirconia particles, 2... Monoclinic zirconia, i! 1th11 Figure'$2121 No. 41! l

Claims (1)

[Claims] 1. Y_2O_3, MgO, CaO, C as a stabilizer
eO_2, Sc_2O_3, Bi_2O_3, Yb_2
A zirconia oxygen sensor element containing at least one type of O_3, characterized in that a monoclinic zirconia layer is provided on the surface thereof. 2. A zirconia sintered body according to claim 1, characterized in that the amount of monoclinic crystals in the surface layer is 10% or more greater than the amount of monoclinic crystals in the interior, as calculated by the following formula. (Formula); Monoclinic proportion (%) = (IM(111)+IM(11■)/ITC(111
)+IM(111)+IM(11■))×100 However, IM(■11) is the peak intensity from the (11■) plane by X-ray diffraction of monoclinic zirconia, and IM(111) is the peak intensity of monoclinic zirconia. The peak intensity from the (111) plane by X-ray diffraction, ITC (111), is the overlapping X-ray diffraction peak intensity from the (111) plane of tetragonal and cubic zirconia. 3. The zirconia oxygen sensor element according to claim 1, wherein the monoclinic zirconia layer on the surface portion has a thickness of 2 mm or less.